The heart of the reactor is the core which is an assembly of "fuel rods' and control rods. The fuel (U-235, PU, etc) is an unstable isotope (or mix) which decays, releasing energy and particals. The particals smash into other atoms of fuel, releasing energy and particals, which smash into more fuel atoms, releasing more energy and particles.....and so forth: a chain reaction. Left uncontrolled, the fuel will continue to consume itself at an increasing rate. In order to allow control of this process, 'moderators' are used, primarily fuel rods and water. The moderator (a stable substance) intercepts particles of decay and limits the chain reaction. Water removes the heat as well (which is used to generate electricity).
The fuel rods and control rods are designed to fit together in a precise geometry to limit/control the chain reaction ongoing in the fuel. Note: This process cannot be "turned off". The chain reaction continues but is reduced to a low, manageable level by the control rods and water (moderators) and there is still heat produced which must be continuously removed.
Meltdown/partial meltdown:
If part of this assembly is damaged (melted), the precise geometry of a portion of the assembly is destroyed. The control rods' role as a moderator is diminished in this area and precise control of the chain reaction is lost. Depending on the level of damage, the chain reaction can build on itself resulting in further damage to intact parts of the core....and so-forth. Meltdown commences. If the damage is slight, continuing to supply water to the core will cool the damaged part of the core and hopefully moderate the chain reaction. The chain reaction in the damaged part of the core cannot be stopped, only prevented from further damaging the intact parts of the core assembly; a standoff. In this case, boron is being added to the water to increase its effectiveness as a moderator.
While this is a greatly simplified explanation, I hope it helps folks understand the fix these folks are in. This is a 'fire' that cannot be easily extinguished, only, hopefully, prevented from spreading.
- Posted by Ghung - The Oil Drum Ghung on March 13, 2011 - 12:14pm
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